Without changing balls it sounds like you have done everthing you can do... changing lines, changing hand postions, changing speed as much as you can.
one of the tricks I have tried in the past is to literally target a line to try to hit the 3-6 or 6-10 'pocket' just to see if I am subconsiously not throwing the line I intend on the first ball.
I have found when I get to the more extreme ends of conditions, very heavy oil or fried heads my muscle memory works against me.
Because I am trying to alter from my standard shot(A game) by too much. example trying to throw with ~20 rotation instead of my usual ~75 it will work for a bit then when I don't concentrate on it I'll throw with 40-45 because I am used to more rotation than I am trying to throw.
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